A mixed solution containing multiple substances undergoes a complex transformation during the freezing process. Initially, some of its components begin to crystallize, leading to a change in the concentration of the remaining liquid. As the temperature continues to drop, there comes a point where the liquid and the solid formed have the same composition. At this stage, the solution is referred to as a eutectic solution. The specific temperature or temperature range at which this occurs is known as the eutectic point or eutectic region. This temperature is also called the full cure temperature, representing the highest temperature at which the material transitions from a liquid to a solid state during cooling.
In the context of freeze-drying, the eutectic temperature is critical. It marks the upper limit for pre-freezing, and the pre-freezing step should ideally be conducted 10–20°C below this value to ensure complete solidification. If not, residual liquid may remain, potentially compromising the quality of the final product.
Another important parameter is the co-melting point, which refers to the temperature at which the solid mixture begins to melt and form a liquid phase upon heating. This is the lowest temperature at which liquidity starts to appear in the solid state during warming. During the drying phase, the temperature of the frozen layer must remain below this co-melting point to prevent premature melting and structural damage.
The eutectic point can be determined using several methods, such as electrical resistance measurement, differential thermal analysis, direct observation with a cryogenic microscope, or numerical calculations. During freezing, the movement of ions slows down as the temperature decreases, causing an increase in electrical resistance. Once all the water has frozen, the ions are no longer mobile, and the resistance jumps sharply. This sudden change is used to identify the eutectic point.
The collapse temperature is another key factor in lyophilization. As ice crystals sublimate during the drying process, the spaces they occupied become voids, creating a porous, sponge-like structure in the dried material. However, this structure is temperature-sensitive. When the temperature rises, the rigidity of the solid matrix decreases. If it reaches a critical point, the structure may collapse, closing off the vapor diffusion channels. This critical temperature is known as the collapse temperature.
Lastly, the glass transition temperature plays a significant role in freeze-drying, especially in pharmaceutical applications. When a liquid is cooled, it can either crystallize or form an amorphous (glassy) state. The glass transition temperature refers to the point at which the material transitions from a supercooled liquid to a rigid, glassy solid. In lyophilization, it's crucial that the product remains below this temperature to maintain structural integrity. However, complete vitrification (fully glassy state) is rarely achieved due to practical limitations in cooling rates. Instead, the glass transition temperature considered here relates to the maximum frozen concentrate, which occurs when the remaining solution becomes highly concentrated and no longer forms ice crystals.
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